a 12-6 pitch is exactly what it sound like. A pitch that drops directly down. Adam Wainwright might have one of the best knuckle curves in baseball. Pitch goes upward out of the hand and has tight spin that brings the ball straight back down. Mike Mussina had a hell of a knuckle cruve that broke straight down.

Not all curveballs are the same it depends on the release. Some are thrown slightly off the side which is going to cause spin that is slightly off center and the ball will break low and to the opposite side of the pitcher's arm side. This is more of a slurve.

Other pitches can break downward, but a "true" 12-6 breaking ball is one that has tumbling over-the-top spin. If it breaks off to the side, it has been released on the side as opposed to directly over the top.

All these numbers being thrown around are referring to the plane on which the ball breaks on a hypothetical clock. A cutter from a right handed pitcher is going to break side to side away from the pitcher's arm side. Meaning a right handed pitcher that throws this pitch will have a break on it that breaks away from a right handed hitter. A lot of times they will start this pitch inside on a right handed hitter and it will cut towards the inside corner in the hope that it will come all the way back to the strike zone. hence the 3-9. Start out from a right handed pitcher at 3 and work straight across to 9.